In the past, various methods have been proposed for sealing a vacuum structure, for example, a vacuum bottle comprising metal-made inner and outer shells as described in Japanese Patent Publication No. 61-1136, Japanese Patent Laid-open Publication No. 2-286111, Japanese Utility Model Registration No. 2502403, Japanese Utility Model Publication No. 7-19402, and Japanese Patent Laid-open Publication No. 7-298991.
(1) In the method of Japanese Patent Publication No. 61-1136, an evacuation port is formed on a bottom plate of an outer shell. A brazing material is disposed on the periphery of the evacuation port in a space between the inner and outer shells and a sealing plate is disposed on the brazing material. After aforementioned space is evacuated, the brazing material is melted by heating, so that the periphery of the evacuation port and the sealing plate are joined together for sealing the evacuation port.
(2) In the method of Japanese Patent Laid-open Publication No. 2-286111, an evacuation port is formed on the outer shell and a brazing material is placed on the periphery of the evacuation port so that the evacuation port can be sealed by the brazing material only and without using a sealing plate. In this publication, there is also disclosed a method such that an upwardly open semi-spherical recess is provided on the outer surface side of the outer shell, with the evacuation port formed centrally of the bottom of the recess so that molten brazing material can be guided into the evacuation port.
(3) In the method of Japanese Utility Model Registration No. 2502403, an upwardly open recess is provided on the outer surface of the outer shell, with an evacuation port provided in the recess, and a brazing material is placed on the periphery of the evacuation port so that the evacuation port can be sealed with the brazing material alone in the same way as in the publication (2) above, without using a sealing plate. Specifically, in the publication the recess is semi-spherical or the bottom of the recess defines an inclined surface with the evacuation port formed at the lowermost end thereof for guiding the molten brazing material to the evacuation port.
(4) In the method of Utility Model Publication No. 7-19402, an evacuation port is provided on the outer shell or inner shell, with a cylindrical wall formed along the inner periphery of the evacuation port which projects on a spatial side, and a brazing material is disposed on the outer periphery of the evacuation port on the opposite side to the cylindrical wall so that the molten brazing material is caused to enter into the cylindrical wall thereby to seal the evacuation port.
(5) In the method of Japanese Patent Laid-open Publication No. 7-298991, first, a component member, that is, outer shell or inner shell, is brazed with a brazing material. Then the brazing material and the component member are formed with a through hole having a diameter of about 1 mm to 3 mm. After a space between the outer shell and the inner shell is evacuated via the through hole, the brazing material is melted by a laser beam for sealing the through hole.
However, with the sealing method described in the publication (1) above, disadvantage is that no provision is made for positioning and holding the brazing material and the sealing plate, so that there is a possibility that the brazing material may be moved from its original position by a time when the evacuation port is sealed by the heated and molten brazing material and the sealing plate, with the result that the brazing material may go out of position and fall in the process of sealing. This poses a problem that the stability of the seal is rather low. Since the brazing material and sealing plate are disposed on the spacious side before evacuation, the condition of their arrangement prior to the evacuation cannot be recognized. Further, since the periphery of the evacuation port and the sealing plate must be joined with brazing material, the areas to be joined together are rather extended and possibly this may lead to unsatisfactory sealing.
In the methods described in the publications (2), (3) and (4) above, as in the case of the publication (1), no provision is made for positioning and holding the brazing material, so that there is a possibility that the brazing material may be moved from its original position by the time of sealing of the evacuation port, with the result that the brazing material may go out of position and fall. Further, since the brazing material is guided to the evacuation port through a curved portion of the recess, the possibility of occurrence of unsatisfactory seal will increase unless the direction of positioning the brazing material is assuredly determined. This poses a limitation such that high precision is required with respect to the design of the production line.
In the method described in the publication (5) above, the component member is brazed with the brazing material and thereafter the brazing material and the component member are bored therethrough to define the evacuation port. Therefore, it is possible to eliminate the possibility of brazing material dropping before the evacuation port is sealed as in the case of methods described in the publications (1) through (4) In such a method, however, since brazing with the brazing material is carried out before the evacuation port is formed, it is difficult to eliminate the gas contained in the brazing material and this easily leads to the occurrence of voids on the fused surface of the brazing material and the component members. Therefore, when a through hole (evacuation port hole) is formed in such a condition and if, after evacuation, the through hole is sealed by fusion, voids cannot completely be eliminated, which leaves the possibility of occurrence of unsatisfactory sealing. Further, in case that a void should occur, the void cannot be visually determined, and this prevents adoption of any correcting measure prior to the stage of sealing, which in turn affect productivity.